1. Field of the Invention
The present invention relates to a switching power supply device installed in the interior of electronic equipment.
2. Description of the Related Art
Conventionally, power supply devices that are incorporated in electronic equipment, such as personal computers, use a switching power supply device. The switching power supply device is one type of stabilized (regulated) direct current power supply, and is configured so as to output a direct current at a constant voltage by converting a direct current obtained from a power supply, such as a commercial power source or a storage battery, into a pulse voltage having a frequency higher than the audio frequency (about several hundred kilohertz) by a high-speed switching function of semiconductor devices such as transistors, and controlling the pulse width and the pulse interval of the pulse voltage. The switching power supply device thus configured has such features as being capable of outputting a constant output voltage at all times regardless of the fluctuation in power consumption of the load, relatively small in size and weight, and moreover highly efficient. With these features, the switching power supply device has conventionally been used particularly suitably for various information devices and communication devices that incorporate central processing units (hereinafter abbreviated as “CPUs”).
FIG. 19 shows schematic views illustrating the configuration of one example of a conventional switching power supply device. FIG. 19A is a side view of the conventional switching power supply device, and FIG. 19B is a top view of the conventional switching power supply device. As shown in FIG. 19, a conventional switching power supply device 1000 has a transformer T, arranged at the center of a printed circuit board P having a plurality of wiring lines configured to form a predetermined electronic circuit. A switching-control circuit U1, switching elements Q1 and Q2, diodes D1 and D2, a capacitor C1 are arranged at respective predetermined positions on the printed circuit board P. The transformer T, the switching-control circuit U1, the switching elements Q1 and Q2, the diodes D1 and D2, and the capacitor C1 are arranged so as to be electrically connected to the wiring lines provided on the printed circuit board P, whereby the switching power supply device 1000 is configured. In the switching power supply device 1000 thus configured, pulse voltage generated by the high-speed switching effect of the switching elements Q1 and Q2 is applied to a primary winding of the transformer T. At this time, an alternating current corresponding to the voltage change of the pulse voltage applied to the primary winding is induced in a secondary winding of the transformer T. Then, the induced alternating current is rectified by the rectification effect of the diodes D1 and D2, and further its ripple is suppressed by the capacitor C1; thus, the obtained direct current is output from an output terminal of the switching power supply device 1000. At this time, the switching-control circuit U1 operates in order to keep the output voltage constant by controlling the ON time and OFF time of the switching elements Q1 and Q2 according to fluctuation of the output voltage. Thus, by using the switching power supply device 1000 that operates in this manner, it becomes possible to supply constant voltage direct current to a fluctuating load at all times. It should be noted here that the conventional switching power supply device 1000 illustrated in FIG. 19 as an example employs a configuration in which all the electronic components for constituting the switching power supply device 1000, such as the transformer T, the switching-control circuit U1, the switching elements Q1 and Q2, the diodes D1 and D2, and the capacitor C1, are arranged on a single printed circuit board P (for example, see Japanese Patent No. 3196187).
In recent years, the tendency in the conditions of electric power supply to CPUs incorporated in electronic equipment including information devices such as personal computers, and communication devices such as mobile telephones, has been toward lower voltage and higher current. For this reason, the switching power supply devices to be incorporated in the above-mentioned information devices and communication devices have also been required to fulfill such a specification that a direct current output at a high current can be obtained. In order to meet such a demand, it is necessary to increase the electric current flowing through the primary winding of the transformer arranged in the switching power supply device. In addition, in order to transmit such a high-current high-frequency switching current and a high-current alternating current induced in the secondary winding of the transformer more efficiently, it is required that wiring lines on the printed circuit board be formed wide. More specifically, a larger width and a shorter length than in conventional devices are required in forming the wiring lines electrically connected to the switching elements, the transformer, the rectifier element, the smoothing element, and the like formed on the printed circuit board. This is necessary to minimize the adverse effects in wiring lines such as parasitic inductance and skin effect and to minimize the power loss within the switching power supply device.
Meanwhile, when the current of the electric power supply from a switching power supply device to a CPU is large, it becomes necessary to take into consideration the effect of voltage drop or the like in the wiring that electrically connects the switching power supply device and the CPU. In order to minimize the effect of the voltage drop or the like in the wiring that electrically connects the switching power supply device and the CPU, it is necessary to arrange the switching power supply device and the CPU so that they are close to each other. More specifically, the wiring line length of the wiring that electrically connects the switching power supply device and the CPU needs to be as short as possible. In order to meet such requirements, the switching power supply device must have as small a size as possible.
Nevertheless, the conventional switching power supply device 1000 employs the configuration in which the constituting elements of the switching power supply device 1000, such as the transformer T, the switching elements Q1 and Q2, the rectifier elements D1 and D2, the capacitor C1, and the switching-control circuit U1, are arranged two-dimensionally on a single printed circuit board P. Accordingly, in cases where wiring lines are formed on a printed circuit board P so as to be sufficiently wide for transmitting a high current high-frequency switching current efficiently as described above, the two-dimensional package density of the electronic components or the like decreases especially in the region in which the switching elements Q1 and Q2, the transformer T, the diodes D1 and D2, the capacitor C1, and so forth are arranged; therefore, it is necessary to increase the size of the printed circuit board P according to the decrease in the package density. Moreover, the conventional switching power supply device 1000 has unused space on the upper face of the printed circuit board P because electronic components with different sizes are arranged thereon together. That is, the configuration of the conventional switching power supply device 1000 has a problem that it is very difficult to reduce the size of a switching power supply device that is highly efficient and capable of outputting a direct current at low voltage and high current.